1. Field of the Invention
The invention relates to a process for the preparation of crosslinked organopolysiloxanes by reacting organosilicon compounds which have olefinic unsaturated groups with organosilicon compounds which have SiH groups in the presence of catalysts and their use as active ingredient in abhesive coating materials for two-dimensional carriers, especially for paper and plastic films.
2. Description of the Prior Art
The preparation of crosslinkable organopolysiloxanes by reacting organopolysiloxanes, which have olefinic unsaturated groups, with polysiloxanes which have SiH groups, in the presence of catalysts and especially platinum catalysts is well known. Those reaction products in which the olefinic unsaturated hydrocarbon group, for example, the vinyl group, is linked over an SiC bond with the polysiloxane backbone, have proven to be particularly stable to hydrolysis. Such vinyl polysiloxanes are, however, very expensive to produce, because they must be synthesized by hydrolysis from halogen vinyl silanes, which themselves can be synthesized only with difficulty and are therefore expensive.
More easily accessible are those polysiloxanes with alkenyloxy groups, for example, allyloxy groups, which can be obtained by reacting chloropolysiloxanes and/or alkoxypolysiloxanes with allyl alcohol or derivatives of allyl alcohol. In so doing, products are obtained having, for example, structural units of the following formula: EQU .tbd.Si--O--CH.sub.2 --CH.dbd.CH.sub.2.
These compounds also may be reacted with polysiloxanes containing SiH groups in the presence of the conventional catalysts at elevated temperatures. This reaction is, however, disadvantageous in that the starting compounds which contain the alkenyloxy groups, as well as the crosslinked organopolysiloxanes, are relatively unstable to hydrolysis, as a result of the SiOC bond.
With this problem in mind, U.S. Pat. No. 4,079,037 discloses a method to improve the hydrolysis stability of alkenyloxy groups linked to silicon over oxygen by specific substitution, especially at the carbon atom which is next to the oxygen of the alkenyloxy group. As a result, the hydrogen atoms of the .alpha.-carbon atom of the alkenyloxy group are replaced totally or partially by hydrocarbon residues with 1 to 6 carbon atoms. At the same time admittedly, the hydrolysis stability of the alkenyloxy-substituted polysiloxanes increases. However, the reactivity of the correspondingly substituted derivatives of the allyl alcohol with halogen polysiloxanes decreases. Thus, as shown in Example 1 of U.S. Pat. No. 4,079,037, after a reaction time of 48 hours, only a 50% yield of alkenyloxy-substituted polysiloxane is obtained. Industrially, such a process is not economically feasible.